Thermomagnetic properties at the nanoscale

Resumo

This thesis deals with the magnetic properties and magnetocaloric effect (MCE) of nanostructured systems. The motivation to carry out such study is based on the unusual magnetic properties that common magnetic materials exhibit in nanoscaled dimensions, often radically different and/or enhanced with respect to their bulk counterparts. The origin of these special properties is found on the reduced dimensionality: when the size of the material reaches the order of nanometers the influence of the surface atoms becomes very comparable (or even higher) than the bulk contribution, the defects due to the broken symmetry may be of significant importance, and other physical effects may also become very relevant when the size reaches the order of characteristic length scales of the material (as for example the domain size). The properties observed in such reduced dimensions are strongly sensitive to small size, shape, and composition variations, what defines the different magnetic structures (nanoparticles, nanowires, thin films and multilayers) as forming specific research fields their differentiated proper characteristics. These new properties have a wide range of technological applications, ranging from magnetic recording to biomedicine. In particular, the study of the MCE in these low-dimensional systems is of great importance both for refrigeration purposes of micro- and nano-electromechanical systems, and for biomedical applications as magnetic agents for hyperthermia treatments. Characterizing the magnetic properties in these reduced dimensions constitutes a complex task due to its strong dependence on different parameters as anisotropy and size distribution, which masks the physical origin and difficulties its characterization: the high parameter-dispersion degree found in real systems, arising from the large dispersion of parameters (size, anisotropy, shape, etc) and uncontrolled interparticle interactions, results in an intricate physical problem non-solvable by analytical methods. In order to investigate this complex scenario it is very helpful the use of computational techniques, which allow for a high control degree of the parameters of the system. So, in this thesis we use a Monte Carlo (MC) method based on the Metropolis algorithm to undertake the study of such magnetic nanostructures: on the one hand, with a MC method the characteristics of the system are perfectly controlled and, on the other hand, we can study problems with no analytical solution, as the magnetic dipole-dipole interaction, for example. Summarizing, the main objective of this thesis is the study of different nanostructured magnetic systems employing the MC technique, as randomly distributed nanoparticles systems or chain-like nanoparticle ensembles, and to investigate how the different parameters characterizing the system (magnetic anisotropy, size, shape, interparticle interactions, etc) rule its magnetic behaviour. This knowledge is then applied to search for the optimizing MCE-based applications, both for hyperthermia and magnetic refrigeration purposes.